The present invention relates to a mass flow controller, commonly termed MFC. More particularly, the present invention provides a novel technique including a device and method for maintaining a fluid flow rate of fluids used in, for example, semiconductor processing or the like. Merely by way of example, the present invention is illustrated using a device and methods related to integrated circuit processing. But it will be recognized that the present invention also can be applied to the manufacture of products such as flat panel displays, hard disk drives, and others.
In the manufacture of semiconductor integrated circuits, process complexity and wafer size tend to increase with time. For instance, wafer size has increased from one inch up to six inches over the past thirty years. Larger sized wafers such as eight inch are now being used. Twelve inch wafers and larger are being proposed. As wafer size and complexity of processing increase, gases used for the manufacture of the integrated circuits also become more important. In particular, control of a selected flow rate range for a process step (e.g., plasma etching ("PE") and chemical vapor deposition ("CVD")) becomes rather important. Accordingly, mass flow controllers have been used to selectively control fluid flow rates of selected process steps.
Conventional MFCs generally have a main flow chamber, or bypass, and a sensing assembly. A sensing assembly is a part of the mass flow controller that measures, or senses, a typically small amount of the total gas flowing through the MFC. The amount of gas flowing through the sensing unit can be calibrated to the total gas flow through the MFC. MFCs use the thermal properties of a gas to measure the mass flow rate. The basic principle is that each gas molecule has a specific heat capacity, which is generally insensitive to changes in temperature or pressure.
The sensing assembly typically includes a relatively thin sensor tube though which the gas flows, a heater to heat the gas in the sensor tube, and a temperature measuring devices, or sensors, deployed on either side of the heater to provide a differential temperature reading that corresponds to the heat transport of the mass of a particular gas flowing through the sensor tube.
Accuracy and response of the sensor assembly is important for good flow control. For accuracy, it is desirable that the differential temperature reading arise from the heat transferred through the sensor tube by the fluid, and not arise from other sources, such as heat traveling down the sensor tube, heat conducted from the heater to the sensor through the media surrounding the sensor tube, or heat arriving at the sensor from another source, such as the bypass. The response of the sensor assembly relates to the speed with which the sensor assembly heats up or cools down after a change in heater power. Quicker response allows the flow to be controlled within finer limits. Some MFCs have added insulation around the sensor assembly to reduce the effects from external heat sources, but such insulation adversely affects the response of the sensor assembly.
The heater and temperature sensor are both typically made by winding a wire around the sensor tube to form a heater coil and sensor coils. Electric current flowing through the heater coil heats the sensor tube and the gas inside the sensor tube. The flow of gas from the heated zone of the sensor tube to the downstream sensor coil creates a temperature differential between the downstream sensor coil and the sensor coil that is upstream from the heater.
Typical sensor coils operate on the principle that the resistance of a metal increases with increasing temperature. The sensor coils are connected to electronic circuits that measure the resistance of each coil. The longer and thinner the wire of a sensor coil is, the higher the coil's resistance will be and, generally speaking, the greater the coil's temperature sensitivity will be. Increased temperature sensitivity allows more accurate and more stable control of the gas flow, so it is desirable to make the coils out of fine wire. However, it is difficult to make reliable electrical connections to fine wire, as such wire may break, either during assembly or during use, for example from vibrational stresses.
From the above, it is seen that a MFC with an improved sensing element that is easier to make, more reliable, more accurate, or more responsive is desirable.